The present invention relates to an improved method and composition for developing a relief image by imagewise exposing a photopolymerizable layer to actinic radiation and subsequently removing the uncured material of the photopolymerizable layer by use of a developer composition to produce the relief image.
In the following description and claims, the term "cured" and derivatives thereof shall be used to describe material obtained by either (1) the photopolymerization of monomeric or low molecular weight compounds, or (2) the crosslinking of photocrosslinkable polymers. The term "uncured" and derivatives thereof shall be used to describe material which has not undergone either photopolymerization or photocrosslinking.
The use of actinic radiation, such as ultraviolet light, to form a relief pattern or image by the imagewise curing of a photopolymerizable layer is well known in the art and is typically used in preparing flexographic printing plates and relief patterns on semiconductor layers. These relief images or patterns are formed by placing an image-bearing process transparency, for example, a line or half-tone negative, over the photopolymerizable layer. The negative is designed to transmit actinic radiation through certain portions of it and to absorb or reflect actinic radiation at other portions so that the photopolymerizable layer underneath the negative is cured imagewise according to the portion of the negative that transmits the radiation. A transparent protective film, called a slip film, usually a polyamide resin film, is commonly provided between the negative and the photopolymerizable layer in order to prevent the negative from damaging the photopolymerizable layer.
After the exposure to actinic radiation, the slip film must be removed from the photopolymerizable layer and the uncured material of the photopolymerizable layer must be removed in order to produce the desired relief image or pattern. Typically, a solvent or developer is used which will solvate the portion of the photopolymerizable layer that has not been cured by the actinic radiation, but will not substantially affect the portion of the photopolymerizable layer that has been cured by the actinic radiation. Typical developers used in such a process are chlorohydrocarbons, such as chloroform, trichloroethylene, tetrachloroethylene or 1,1,1-trichloroethane, either singly or in a mixture with a suitable lower alcohol, such as n-butanol; saturated cyclic or acyclic hydrocarbons, such as petroleum ether, hexane, heptane, octane, cyclohexane or methylcyclohexane; aromatic hydrocarbons, such as benzene, toluene or xylene; or lower aliphatic ketones, such as acetone, methylethyl ketone or methylisobutyl ketone.
In using the prior art developer compositions described above, it usually takes a disadvantageously long time to remove the portion of the photopolymerizable layer which has not been cured. Moreover, the cured relief patterns or images swell or distort during the washing or removal of the uncured portion of the photopolymerizable layer by the developer composition. Further, under microscopic inspection, the printing surface may exhibit a nonsmooth surface due to cracks, crevices, bumps and other surface irregularities and this may detrimentally affect the printing quality. Additionally, these prior developers are toxic and must be subjected to special handling procedures and some have low flash points which makes very hazardous the separation of them from the removed or washed-out, uncured, photopolymerizable material.
U.S. Pat. No. 4,806,452 discloses a developer composition which contains, as an essential component, at least one monoterpene and up to 40% by weight of an additional solvent.
Flexographic photopolymer printing plates can be made of a variety of different materials, including:
1. block copolymers of styrene and butadiene or styrene and isoprene (U.S. Pat. Nos. 4,323,636, 4,323,637, 4,423,135 and 4,369,246), PA1 2. a mixture of nitrile rubber, an addition photopolymerizable tri- or tetra-unsaturated ester of acrylic or methacrylic acid and an addition polymerization initiator activated by actinic radiation (British Patent No. 1358062), PA1 3. a mixture of a high molecular weight butadiene/acrylonitrile copolymer containing carboxyl groups, a low molecular weight butadiene polymer which may or may not contain carboxyl groups, an ethylenically unsaturated monomer and a free-radical generating system (U.S. Pat. No. 4 177 074), and PA1 4. a mixture of a high molecular weight butadiene/acrylonitrile copolymer containing carboxyl groups, a high molecular weight butadiene/acrylonitrile copolymer which does not contain carboxyl groups, an ethylenically unsaturated monomer and a free radical generating system (U.S. Pat. No. 4,517,279). The developer compositions needed for development of these different types of plates vary depending on the specific composition of the plate employed. For example, a developer suitable for the development of Flex-Light type FL-SKOR and Flex-Light type FL-KOR photopolymer plates (registered trademarks of W. R. Grace & Co.) may not be effective in the development of Flex-Light type FL-1 and Flex-Light type FL-AL photopolymer plates (registered trademarks of W. R. Grace & Co.).
U.S. Pat. No. 4,847,182 discloses a developer composition comprising a terpene which can be blended with a nonsolvent, such as benzyl alcohol, which nonsolvent is effective to promote the spontaneous separation of uncured material from the developer solution after the plate has been developed. Emulsifiers, such as polyethylene glycol mono(nonylphenyl) ethers, can be incorporated in the developer composition so that the plate can be rinsed with water prior to oven drying.
It is an object of the present invention to provide a developer composition for use in the preparation of a relief image or pattern by photopolymerization in which the washout time and the drying time are substantially shorter in comparison with conventional processes, the photopolymerized relief images or patterns do not undergo significant permanent distortion or swelling during washout with the developer composition and the developer composition can be economically recovered from the uncured photopolymerizable material without special handling procedures and without significant risk of explosion.
It is a further object of the present invention to provide a developer composition which has a low toxicity and volatility, is biodegradable and can be used for developing relief images or patterns produced on a wide variety of types of photopolymer plates.
It is a still further object of the present invention to provide a developer composition which will dissolve the slip film during the development of the plate and thereby eliminate the step of removing the slip film from the photopolymer plate.
It is a still further object of the present invention to provide a developer composition which enables the developed photopolymer plate to be rinsed with plain water thereby eliminating the tackiness produced in the formed image or pattern by the conventional solvent rinses and allowing the easy disposal of waste rinse solution.